The present invention relates generally to optical communications devices, and more specifically to a computer bus based on such devices.
The speed and complexity of electronic systems is constantly increasing, brought about in large part by the increasing speed and complexity of available processors. Associated with this processing power, a functional unit based on one of these new processors may accept large amounts of input data and provide large amounts of output data. In addition to advances in processors, architectures have been migrating to distributed computing environments, with many functional units connected to a bus. The high-speed data input and output associated with such processors require that the bus have a very high throughput. Conventional electronic bus implementations use large numbers (typically 32 or 64) of parallel electrical lines for the physical interconnection medium (the "backplane"). At high clock speeds (&gt;25 MHz), such buses exhibit serious problems associated with crosstalk and losses.
Advances in processors have been augmented by the development of advanced packaging techniques based on multichip modules (MCMs), where multiple chips are mounted to a common substrate and incorporated into a single package. Due to the large total chip area which can be contained in an MCM, it can have very substantial processing power. Thus, where functional units on a bus are based on MCMs, the demands on bus throughput can be even greater.
Optical buses can potentially overcome the limitations of electronic buses since optical interconnections can achieve virtually unlimited bandwidth (&gt;&gt;10 Gb/sec) over large distances (&gt;&gt;1 km). A number of optical buses have been proposed. These buses typically require that light be launched into the bus and tapped out of the bus where each functional unit (node) is attached to the bus. The bus taps are typically devices known as passive directional couplers.
By way of brief background, a directional coupler is a four-port device having a region (referred to as the interaction region) where two single-mode optical waveguides are brought close together. An optical signal in one waveguide can be coupled to the other waveguide through evanescent field coupling. This is based on the fact that there is a portion of the optical field outside of the waveguide (the "evanescent" field) which decays exponentially away from the edge of the waveguide. If another waveguide is brought sufficiently close to the first one, it will intersect some of the evanescent field and light can be coupled from the first waveguide to the second (and vice versa). This coupling phenomenon is analogous to quantum mechanical tunneling. The degree of coupling depends on the proximity of the waveguides, the length over which the waveguides are in proximity, and the indices of refraction of the materials.
By way of additional background, a fiber optic Mach-Zehnder interferometer is a four-port device comprising a first 3 db directional coupler (a 50% splitter) between the first and second ports on one side and first and second legs on the other side, and a second 3 db directional coupler between the first and second legs and the third and fourth ports. Light incoming to the first port is split equally between the first and second legs; the relative amounts of light exiting the third and fourth ports depend on the optical path difference between the two legs. By varying this optical path difference, the light can be selectively switched or split as desired.
For a directional coupler to be used as a bus tap, the geometrical and optical parameters would typically be chosen to provide a small degree of coupling into the node, with most of the light continuing to propagate down the bus. Buses based on directional couplers tend to be unidirectional in nature. Since bidirectionality is normally required in typical computer applications, the use of an optical bus might well militate toward a folded configuration or a ring topology, which may be undesirable. Additionally, since conventional tapped optical buses require a laser at each node for each bus line, high reliability may be difficult to achieve since lasers tend to be among the least reliable components in the bus. This problem can be mitigated by using a serial communication protocol on a single bus line, but that typically adds electronics overhead, and may be undesirable for other reasons.
Conventional tapped optical buses tend to have a number of additional problems. For example, a bus with N nodes attached may experience an optical loss between the transmitting laser and the receiver which scales as 1/N.sup.2. Furthermore, in such buses, the received signal power level is dependent on which node is transmitting data onto the bus. This results in the need to adjust the automatic gain control (AGC) at each receiver, which is time consuming and slows the bus down.
Therefore, while the concept of an optical bus shows great promise, the great success of fiber optic technology in the field of long-distance communications has not been paralleled or even approached in the more localized realm of computer systems.